It happened when I open a book via shortcut menu. Rebooting with factory-default logo and stucking on this screen. Nothing response but power button. All I can do is just turn it off.

These are what I tried...

- press reset button
- plug to power line.
- plug to power line then press reset button.
- plug to PC
- put off battery and put it in again. (either with or without power line)
- update firmware in the same version (15.3) (completely)

It's still not work. Do anyone have an idea what's going on?

Thank you for advance

UPDATE: I've already solved it by hard reset. But still wonder why it happened.